According to the United Nations Educational, Scientific and Cultural Organization website, documents about the Nanjing Massacre, submitted by the Chinese government, were added this year under the program, which aims to preserve and offer universal access to the world’s documentary heritage.

The Japanese government has acknowledged that the imperial army looted and killed civilians in Nanjing after occupying the city in 1937. But the debate over the number of victims has remained a cause of tension between the two states, with Beijing saying that 300,000 people were killed while Tokyo says it can’t verify the numbers. Conservative politicians and historians in Japan have said they believe the actual number was a small fraction of 300,000.

The authenticity of the documents added to the U.N. list “hasn’t been verified by experts,” Mr. Suga said at his regular news conference, adding that the Japanese government hasn’t been able to confirm the details of the Chinese documents yet. The government will consider suspending its financial contribution to Unesco because it has concerns about how the organization operates, he said.

According to the Unesco website, the documents about the Nanjing massacre consist of three parts including information about the postwar investigation and the trials of war criminals. China's nomination letter said the materials include photos taken by the Japanese army and documentary film shot by an American priest.

Update: Chinese foreign ministry spokeswoman Hua Chunying on Tuesday blasted Japan’s response as “shocking and totally unacceptable," adding that China’s filing of Nanjing Massacre documents met Unesco requirements. “The Japanese side may threaten to reduce its financial contributions to Unesco, but it can never remove the stain of history. The harder it tries to remove it, the darker the stain gets,” she said.